Large boilers, furnaces, and other combustion devices recover heat from their exhaust gas and use it to heat the incoming combustion air. One device in common use on large boilers to recover heat from the exhaust, and transfer it to the inlet air, is the Ljungstrom air heater. The Ljungstrom air heater is comprised of a regenerative drum rotatably mounted in a housing. The drum is divided into separate compartments through which the hot gases and cool air alternatively flow. The drum has the capacity for heat absorption and release. As the drum rotates, it absorbs heat from the hot gas in one compartment and gives up heat to the cooler air in another compartment. Devices on other boilers include simple shell and tube heat exchangers to accomplish the heat transfer.
None of these heat exchangers cools the hot gas below the dewpoint, because the corrosion caused by condensed moisture, and the fouling due to particulate mixed with the moisture, would shortly render the heat exchangers inoperable. In addition, the moist acid gas flowing downstream of the air heater would corrode an electrostatic precipitator and an induced draft fan, both of which are used to further clean and dispose of the exhaust gas. Also, the moisture in the electrostatic precipitator would cause the collected ash to agglomerate and stick to the walls of any hoppers thereof.
Operating with these limitations, the Ljungstrom air heater, on a large utility boiler, will typically accept hot flue gas from the boiler at 630.degree. F. and cool it to 300.degree. F. If the ambient air is 60.degree. F., it will heat that air to 496.degree. F., far short of the 630.degree. F. exit flue gas temperature.
On many small industrial boilers, small furnaces, dryers and other equipment, heat recovery is not practiced at all because of the high capital cost and operating cost of smaller sized regenerators. Where it is practiced, very simple equipment, with limited effectiveness, is employed.
Several inventions to improve waste heat recovery from exhaust gases have been recorded in the art. Fallon, Jr., et al, U.S. Pat. No. 4,083,398, issued on Apr. 11, 1978, envisions a conventional heat exchanger in the exhaust gas duct, and a second conventional heat exchanger in the air inlet duct, with heat transfer between them by a liquid heat exchange medium. Lange, U.S. Pat. No. 3,953,190, issued on Apr. 27, 1976, teaches the countercurrent flow of hot exhaust gas, from a glass furnace, with granular media to heat the media. Lange also teaches particle collection by the media. However, the media is subsequently fed into the furnace after heating. Also, the temperature of the exhaust gas from the glass furnace is not allowed to drop below its dewpoint.
Morris, U.S. Pat. No. 4,012,210, issued on Mar. 15, 1977, teaches the countercurrent flow of exhaust gas and media for particle removal, but without heat transfer. Combs, U.S. Pat. No. 4,053,293, issued on Oct. 11, 1977, teaches the combination of dust collection and heat exchange from exhaust gas, but does so with a conventional tubular heat exchanger and a special settling chamber.
Olsson, U.S. Pat. No. 1,148,331, issued on July 27, 1915, does not practice exhaust gas energy recovery, but has invented a furnace for indirectly heating a process gas. Olsson teaches the heating of a cool gas by countercurrent contact with heated soiled bodies, such as sand, which has been previously heated by countercurrent contact with a hot gas. Olsson further teaches the recirculation of the sand, and the use of the sand to provide the gas seal between the two heat exchanger vessels. Olsson uses freely falling fine sand to accomplish the heat exchange, rather than a slowly moving packed bed of larger media. Olsson makes no mention of the use of heat from condensing water vapor in his heating gas, nor would his invention allow him to do so. Neither would he be able to remove any particulate from the heating gas. In fact, the falling sand would decrepitate, adding to the particle load in the cooled heating gas leaving his invention.
In the field of petroleum refining, the moving bed pebble heater is a well known device used to heat feedstocks. Goins, U.S. Pat. No. 2,774,572, issued on Dec. 18, 1956, teaches the countercurrent flow of pebbles with hot gas to heat the pebbles in a first chamber, the flow of hot pebbles from the first chamber to a second chamber, the subsequent countercurrent flow of the heated pebbles with cool feedstock in the second chamber to heat the feedstock, and the pneumatic recirculation of the pebbles. Clean fuel is burned specifically to heat the pebbles. The advantge of the heater is that any coking that occurs on the heat transfer surface can be easily burned off and will not build up, as it would in a conventional heat exchanger. The feedstock is heated to a maximum temperature of 650.degree. F., so as to limit coking by the pebble heater, and the maximum temperature of the heating gas is over 1,500.degree. F. Hence, the moving bed pebble heater does not teach energy recovery from a waste gas stream. Also, this pebble heater does not teach condensing water from the heating gas, heating with a dirty gas, or the recovery of contaminants from the pebbles.
Moving bed pebble heat exchangers are also used to cool gases and collect condensables in petroleum refining and air separation operations. In these applications, the gas to be cooled is passed through a moving pebble bed in counterflow with cooled pebbles in such a way that impurities condense on the pebbles and are removed from the product stream. The pebbles may be cooled and cleaned externally from the heat exchanger.
Kasbohm, et al, U.S. Pat. No. 3,023,836, issued on Mar. 6, 1962, teaches a creeping pebble bed cooler which uses an alternate purge gas to cool the pebbles and remove impurities, and slowly moves the condensables into the warmer zone by moving the bed. Gifford, U.S. Pat. No. 2,966,037, issued on Dec. 27, 1960, provides external cooling through the wall of the moving pebble bed container. While both Kasbohm and Gifford teach condensation of vapors in moving pebble bed heaters, they do so solely to remove the material as an impurity, and not for purposes of energy release. Further, the condensate is not removed as a liquid in steady state operation from within the heat exchanger, but cooled to a semi-solid state for removal with the media.